1. Field of the Invention
The present invention relates to a method for fabricating an oxide superconducting device (a Josephson Junction device) using an oxide superconductor.
2. Description of the Related Art
Characteristics of a Josephson Junction are high-speed processing and low power consumption. Among these benefits the low power consumption provides a great benefit when a lot of devices are integrated. Reproducibility and uniformity of the device characteristics are necessary in order to realize a desired design performance.
As methods for fabricating a Josephson Junction as the basics of an oxide superconducting device the following methods are known:
(a) a method for fabricating graded-step-type junctions, in which a graded-step with several hundreds of nm height is formed on a substrate using a conventional photolithography technique and an ion beam such as Ar, or using a reactive ion etching, and in which a weak link of an oxide superconducting thin-film introduced at the graded-step portion is utilized; PA1 (b) a method for fabricating grain boundary-type junctions, using a bi-crystal substrate; PA1 (c) a method for fabricating ramp-edge-type junctions, in which a lower superconducting thin-film is first formed and a graded-step is constructed thereon using a photolithography technique; in the graded-step portion, a barrier layer is formed between the lower thin-film and an upper superconducting thin-film formed afterward; and PA1 (d) a method for fabricating plane-type junctions, in which a deteriorated region is formed on a substrate using a converging ion beam (See for example, Japanese Patent Laid-Open No. 6-151986), or a micro groove is formed on a substrate, to utilize a weak link region of an oxide superconducting thin-film generated in the beam irradiation region.
Of these fabricating methods, the method for fabricating grain boundary-type junctions using a bi-crystal substrate mentioned in (b) is not suited for producing an integrated circuit because the position where the Josephson Junction is constructed is limited to the region where the bi-crystal joins together, so that it is difficult to arrange freely a lot of junctions on a substrate. With regard to the method for fabricating ramp-edge-type junctions mentioned in (c) there are disadvantages such as complexity of the fabricating procedure and difficulty in avoiding deterioration in the superconductivity during the procedure. With regard to the method for fabricating graded-step-type junctions mentioned in (a) and the method for fabricating plane-type junctions using a converging ion beam mentioned in (d) the procedure is simple and the degree of freedom for arrangement of junctions is high. The method mentioned in (d) using a converging ion beam is more advantageous than the method mentioned in (a) using photolithography when realizing an integrated circuit, because the method mentioned in (d) makes it possible to construct junctions in a smaller micro region than the method mentioned in (a).
Referring to the drawings, a method for fabricating junctions by forming a groove on a micro region of a substrate using a conventional converging ion beam will be described.
FIG. 9 is a plan view showing a diagrammatic sketch of the construction of a conventional junction. FIG. 10 is a cross-sectional view of the conventional junction taken along the line A-A' of FIG. 9. In FIGS. 9 and 10 numeral 1 indicates a single crystal substrate such as MgO, numeral 2 indicates an oxide superconducting thin-film (device) such as YBa.sub.2 Cu.sub.3 O.sub.7-x (0.ltoreq.X.ltoreq.0.5) or NdBa.sub.2 Cu.sub.3 O.sub.7-x (0.ltoreq.X.ltoreq.0.5), numeral 3 indicates an irradiation region of a converging ion beam, numerals 4, 5, and 6 indicate a grain boundary of the oxide superconducting thin-film 2, numerals 7 and 7' indicate electrodes for extraction. In the irradiation region of a converging ion beam 3 the ion beam etches the substrate 1 to form a V-shaped groove. When the oxide superconducting thin-film is formed on the MgO substrate so as to be oriented in the direction of the c-axis, the film oriented in the direction of the c-axis will grow in such a manner that the c-axis is perpendicular to the substrate on the plane surface of the MgO substrate, and the c-axis is also perpendicular to an inclined surface of a V-shaped groove portion. As a result, a grain boundary in which orientations of the adjacent crystal grains are different from each other is formed, and this grain boundary portion forms weak links at the regions 4, and 6 shown in FIG. 10, where the crystal grain grown on the substrate comes into contact with the crystal grain grown on the inclined surface of the groove, and also forms a weak link at the region 5 shown in FIG. 10, where the crystal grains grown on the inclined surfaces of the groove come into contact with each other, so that a junction is formed.
FIGS. 11A, 11B, and 11C are sectional views showing the fabricating processes of a conventional method for fabricating an oxide superconducting device. Referring to FIG. 11A, a gold thin-film 8 having a thickness of about 100 nm is formed on the substrate 1 and a Ga ion beam is radiated at a junction fabricating portion by means of a converging ion beam apparatus. The gold thin-film 8 is a film to inhibit electrification due to the ion beam. Then, as shown in FIG. 11B, the gold thin-film 8 is completely removed. Since the beam intensity distribution of an ion beam has a Gaussian distribution, as shown in FIG. 11B, a V-shaped groove is formed in the ion beam irradiation region 3 of the substrate 1. Next, as shown in FIG. 11C the oxide superconducting thin-film 2 is formed. Then, a pattern is formed so as to cross over the ion beam irradiation region 3 and the device shown in FIG. 9 is fabricated.
Since the method for fabricating junctions using a converging ion beam has a high degree of freedom for arrangement of junctions and it provides an advantage in its feasibility in micromachining as described above, the feasibility in fabricating an integrated circuit and the like is high. Under the present circumstances, however, because of a wide range of variations in device characteristics such as a critical current of each device and because of poor reproducibility, there is a problem that it is difficult to prototype with high reproducibility a circuit combined with a plurality of devices.
It is necessary to form with high reproducibility an optimized groove configuration for constructing a junction in order to form a device having high reproducibility and uniformity. In a conventional method for fabricating junctions using a converging ion beam, however, it is difficult to optimize the groove configuration, because the beam intensity distribution determines the groove configuration and only a limited configuration is obtained under performance specifications of the converging ion beam apparatus.
Further, since the beam intensity distribution differs according to the apparatus and to the ion source, and the ion source varies over time, a problem arises in the reproducibility of the groove configuration, such as variation in the groove configuration due to a change of the apparatus or replacement of the ion source.
FIG. 12 shows measured data of the sectional configuration of the groove portion of an MgO substrate in a conventional method for fabricating junctions using a converging ion beam. A gold thin-film with a thickness of 100 nm is formed on the MgO substrate and the junction forming portion is irradiated with a Ga ion beam using a converging ion beam apparatus. Acceleration voltage is 30 KeV, beam current is 6 pA, and amount of ion irradiation is 5.12.times.10.sup.17 /cm.sup.2. FIG. 12 shows data measuring the sectional configuration of the MgO substrate by means of AFM after removing the gold thin-film. FIG. 13 illustrates variations of the inclination angle of the inclined portion according to the position of the beam within the groove in FIG. 12.
When the width of the groove is small, growth of a crystal grain whose orientation is the same on a groove as on the substrate is predominant, so that a junction is not formed. An inclined surface having an almost fixed inclination with a certain length is necessary in order that film oriented in the direction of the c-axis may grow ably along the inclined surface of the groove. For example, the inclined surface needs a length not less than several tens of nm in a typical film forming condition of the YBCO film on the MgO substrate.
Referring to FIG. 12, the conventional inclined portion of a groove apparently seems to have an inclined surface with a fixed inclination angle, but actually there is no fixed inclination angle portion as appreciated in FIG. 13. Therefore, although there is no fixed inclination angle of the inclined surface, a crystal grain grows centering in a region with small variations in angle within a range of several tens of nm. For example, referring to FIG. 13, in a and b regions positioned near the center of a groove a crystal grain having an inclination angle of about 18-20.degree. (degree) may grow dominantly. In c and d regions a crystal grain having an inclination angle of about 10.degree. may grow dominantly.
Ideally, a crystal grain grows dominantly in a region having a small variation in inclination angle with large width, so that a crystal grain with a fixed crystal orientation grows over the entire inclined surface, junctions are formed at the positions 4, 5, and 6, illustrated in FIG. 10. As shown in FIG. 13, however, when the dominant angle is not clear, the following cases may occur. One case is that growth of a crystal grain having an inclination angle of about 20.degree. in the central portions of the groove indicated by a and b becomes dominant, and it spreads over the entire groove. Another case is that growth of a crystal grain may occur in a and b regions, and c and d regions with inclination angles that are different from each other. The other case is that growth of a crystal grain of about 15.degree. that donates a degree between the region a and b, and a degree between the region c and d may occur dominantly over the entire groove. Further, there may be a case that these cases described above are mixed according to positions of junctions(vertical direction in FIG. 9).
Although the characteristics of a device depend greatly on the inclination angle of a grain boundary, it is impossible to fabricate a groove configuration with a fixed inclination angle using a conventional fabricating method. This, then, has contributed to the variations in the characteristics of devices. Further, because of the poor reproducibility in forming a groove configuration, the reproducibility in the characteristics of a device is also poor.
In FIG. 12 a V-shaped groove having a width of about 120 nm and a depth of about 15 nm is obtained. When the period of fabrication and the amount of irradiation ion increase, the depth of the groove can become deeper. But since the diameter of the beam determines the width of the groove, the width of the groove hardly changes even if the amount of ion irradiation increases. Therefore, in the conventional method it is impossible to optimize the depth and the width of a groove. This causes the following problems.
When the thickness of a superconducting thin-film becomes large, a crystal grain that grows on an outer side of a groove on a substrate elongates toward the inside from both sides of the groove to cause the groove to be filled, and eventually two crystal grains make contact at a surface portion of the thin-film. In this state, the grain boundary that has an inclination angle to cause a weak link is not formed on the surface portion of the thin-film so that a leak current is caused to flow. In a YBCO thin-film forming condition, when the thickness of the thin-film becomes not less than 300 nm, the number of junctions in which a leak current is dominant increases, the characteristics of the device vary, and reproducibility deteriorates.
In order to avoid this leak current and to realize stable characteristics in spite of a large thickness of a thin-film, it is necessary to extend the width of a groove according to the thickness of the thin-film. Since the junction characteristics depend greatly on the variations in angle at a grain boundary portion, it is necessary to adjust the depth so as to make it deeper according to the width in order to optimize the inclination angle. Taking into consideration the application of an oxide superconducting device to an electronic device, ideally the thickness of a superconducting thin-film must be thicker than a wave length penetrating into a magnetic field. For example, in the case of YBCO the thickness of a thin-film is preferably not less than 300 nm. In a conventional fabricating method, however, it is impossible to adjust the width and the depth of a groove according to the thickness of a superconducting thin-film and to obtain a desirable inclination angle, so that it is difficult to fabricate a device with small variations and with high reproducibility.
Since in the example shown in FIG. 10 three junctions are formed within the groove, the junction represents characteristics such that three junctions are connected in series. The serial connection of the junctions causes problems in some uses of a device. In order to avoid such problems, there is provided a device with only one junction formed in the bottom portion of a groove in such a manner that the inclination angle of the inclined portion of a V-shaped groove becomes gradually smaller toward the periphery of the groove.
FIGS. 14A, 14B, 14C and 14D are diagrams illustrating a fabricating operation and processes showing the method for fabricating the superconducting device described in Japanese Patent Laid-Open No. 7-94790. An MgO substrate 1 is prepared as shown in FIG. 14A, a V-shaped groove 3 is formed using a converging ion beam as shown in FIG. 14B, the V-shaped edge is shaved to become smooth by means of an etching over the entire surface of the substrate using an Ar ion milling apparatus, and a curved portion 9 is formed as shown in FIG. 14C. Finally, as shown in FIG. 14D, a YBCO thin-film 2 oriented in the direction of the c-axis is formed. A device can be fabricated in which only a grain boundary positioned at the center portion of a V-shaped groove determines the characteristics by achieving a radius of curvature in the curved portion so as to cause no grain boundary.
This fabricating method has the following problems. It is difficult to optimize the groove configuration, because the configuration is eventually determined by the beam configuration of the converging ion beam and the etching characteristics of the Ar ion milling apparatus. For example, when the curved portion 9 is processed to a curvature so as to cause no grain boundary, the central portion of the groove is also etched. So it is difficult to maintain an optimal V-shaped groove for a junction. Additionally, it is difficult to form a groove configuration so as to fix an angle of a grain boundary portion with high reproducibility. The reason is as follows; although the characteristics of a device are eventually determined by the angle of a grain boundary at the central portion of a V-shaped groove, the etching method using a converging ion beam apparatus is the conventional method shown in FIGS. 9, 10, and 11.
Further, FIGS. 15A, 15B, and 15C are diagrams illustrating a fabricating operation and process showing the case in which a substrate is processed by radiating a converging ion beam from a slanting direction in such a manner that one side of the inclination angle of the V-shaped groove is obtuse and the opposite side of the inclined angle is acute in accordance with Japanese Patent Laid-Open No. 8-153832. A detailed description will be omitted because it is the same as the case described in FIGS. 11A, 11B, and 11C. An ion beam is radiated from a slanting direction of the substrate as distinct from FIGS. 11A, 11B, and 11C. Junctions can be stably formed by means of an acute angle of a beam irradiation, even if conditions for forming a thin-film or a substrate are changed. In this case it is also difficult to obtain an optimized groove configuration with high reproducibility, since the fabricating method is the same as the conventional method described in FIGS. 9, 10, and 11 except that an ion beam is radiated from a slanting direction.